rustc: Fix the logic for finding the Android main function

I don't understand what this logic is doing

src/test/compile-fail/issue-2995.rs

@@ -11,3 +11,5 @@
 fn bad (p: *int) {
     let _q: &int = p as ∫ //~ ERROR non-scalar cast
 }
+
+fn main() { }

src/test/compile-fail/tag-variant-disr-dup.rs

@@ -19,3 +19,5 @@ enum color {
     black = 0x000000,
     white = 0x000000,
 }
+
+fn main() { }